DE PARTICLE AND FIELD OBSERVATIONS DURING THE POLEWARD EXPANSION OF AN AURORAL SURGE THROUGH THE PLASMA SHEET

A substorm surge on September 25, 1981 was observed by the imagers on the high-altitude DE-1 spacecraft and encountered by the coplanar, low -altitude DE-2 spacecraft. This case is unique among those that we exa mined in that DE-2 traversed the surge during the similar to 10-15 min period of its rapid poleward motion through the plasma sheet. Because of this, the DE-2 measurements provide a critical test for the recent proposal that the substorm expansion phase is due to an anti-sunward- propagating reduction in the large-scale magnetospheric electric field imparted to the magnetosphere from the solar wind [Lyons, 1995]. A sp ecific prediction of this theory is that the electric field reduction in the equatorial magnetosphere maps to the ionosphere as a poleward-m oving electric field reduction. In the ionosphere, large growth-phase electric fields are expected poleward of the active surge aurora, and significantly weaker electric fields are expected equatorward of the a ctive aurora. This electric field pattern is expected to propagate pol eward through the plasma sheet with the surge. The DE-2 measurements s how significant southwestward electric fields within the portion of th e plasma sheet poleward of the active auroral region, and greatly redu ced electric fields within the heated central plasma sheet that was le ft behind by the narrow region of poleward-moving, active aurora. We a lso find weak electric fields equatorward of the region of active auro ra in typical DE-2 passes over auroral surges, where the region of act ive aurora had moved poleward to very near the magnetic separatrix pri or to the satellite pass. These observations agree strongly with the L yons [1995] predictions. Consistent with previous observations, the DE -2 measurements also show strong and variable electric fields within t he region of active surge; however these fields are not specifically a ddressed by the theory.